1. Field of the Invention
The present invention relates to a an active matrix display device, and particularly to an active matrix liquid crystal display device including means for applying gate pulses to transistors connected to pixels composed of a liquid crystal.
2. Description of the Related Art
The general construction of a prior art active matrix liquid crystal display device will be briefly described with reference to FIG. 5. FIG. 5 is a typical equivalent circuit diagram showing an area including one pixel. Each pixel is provided at an intersection between a gate line X and a signal line Y. The pixel composed of a liquid crystal is equivalently indicated at a liquid crystal capacitance C.sub.LC. In general, the liquid crystal capacitance C.sub.LC is connected in parallel to an auxiliary capacitor C.sub.S. In the liquid crystal capacitance C.sub.LC, one end is connected to a driver transistor Tr and the other end is connected to an opposed electrode, from which a specified reference voltage Vcom is applied. The transistor Tr comprises an MISFET type film transistor. A drain electrode D of the transistor Tr is connected to a signal line Y to receive video signals Vsig. A source electrode S is connected to one end of the liquid crystal capacitance C.sub.LC, that is, the pixel electrode. A gate electrode G is connected to the gate line X, and is applied with gate pulses having a specified gate voltage Vgate. A coupling capacitance C.sub.GS is formed between the liquid crystal capacitance C.sub.LC and the gate electrode G. The coupling capacitance C.sub.GS is a combination of a floating capacitance component between the pixel electrode and the gate line X, and a parasitic capacitance component between the source area and a gate area within the transistor Tr. In the coupling capacitance C.sub.GS, the latter parasitic capacitance component is predominant, and which tends to be varied depending on each transistor Tr.
Next, the problem to be solved by the present invention will be briefly described with reference to FIG. 6. When a gate pulse with a voltage Vgate is applied to a gate electrode G in a selected period of time, the transistor Tr becomes in the on-state. At this time, video signals Vsig supplied from the signal line Y is written on the pixel made of a liquid crystal, that is, the so-called sampling is carried out. Then, in a non-selected period of time, the applying of the gate pulses is stopped, and the written video signals are held in the liquid crystal capacitance C.sub.LC. At a transition from the selected period of time to the non-selected period of time, the rectangular gate pulse is shaped into a rapid fall from a high level to a low level. At this time, the charge stored in the liquid crystal capacitance C.sub.LC by means of the coupling through the above-described coupling capacitance C.sub.GS is instantaneously discharged, thus causing a voltage shift .DELTA.V in the video signals Vsig written in the pixel. Since the coupling capacitance C.sub.GS is varied depending on each pixel, the voltage shift .DELTA.V is varied. This results in a disadvantage of generating the so-called rough-feeling on the display screen, resulting in the significant deterioration of the display quality.
In each pixel composed of a liquid crystal, video signals are written in a selected period of time, and the written video signals are held in the subsequent non-selected period of time, to thus constitute one field. The transmissivity of the pixel in one field is dependent on the effective voltage applied to the liquid crystal in the one field. The driver transistor is required to secure the on-current necessary for completing the writing within the selected period of time. Also, in order to obtain the effective voltage enough to lighten the pixel during the one field, the leak current in the non-selected period of time, that is, the holding period of time is reduced as small as possible. The effective voltage is largely affected in the non-selected period of time which is very longer than the selected period of time. Accordingly, the above-described voltage shift .DELTA.V generated in the on-state after charging of the liquid crystal capacitance C.sub.LC is greatly affected by the effective voltage applied to the liquid crystal, thereby damaging the display quality.
Conventionally, for suppressing the absolute amount and the variation of the voltage shift .DELTA.V, there has been proposed a technique wherein an auxiliary capacitor C.sub.S connected in parallel to the liquid crystal capacitance C.sub.LC is formed so as to be larger. Namely, the technique is intended to previously store the charge enough to supplement the charge amount discharged through the coupling capacitance C.sub.GS in the auxiliary capacitor C.sub.S. However, since the auxiliary capacitor C.sub.s is formed in the pixel area, there arises such a disadvantage that, if the dimension thereof is set to be larger, the opening ratio of the pixel is sacrificed, which makes it impossible to obtain the sufficient display contrast.